1. Technical Field
Apparatuses and methods consistent with the present inventive concept relate to energy storage devices, and more particularly to electrodes for energy storage devices.
2. Related Art
Commercial energy storage device electrodes are manufactured by coating a slurry onto a metallic current collector. To fabricate an electrode by a conventional slurry coating process, a binder, for example polyvinylidene fluoride (PVDF), is pre-dissolved in a solvent to form the binder solution. A widely used solvent for energy storage device manufacture is N-methylpyrrolidine, also known as NMP. The conductive carbon active material and binder solution are mixed to form the slurry. The slurry is then coated onto a current collector by a cylindrical roller. The current collector along with the coating layer is passed through a long dryer, where the solvent is dried and removed from the electrode.
Binding mechanisms for powder particles to powder particles may be different than binding mechanisms for powder particles to a metal current collector. In the coating process, in order to form strong bonds between the powder particles and between the powder particles to the metal current collector, a high percentage of the binder, for example greater than 10% of total solids, is used since the same binder (or binders) has (have) to bond two different types of materials (i.e., the powder particles and the metal current collector. A high percentage of binder means that less active materials can be added to the electrode formulation. The higher percentage of binder blocks the surface area of the active materials, thus lowering the energy density of the device. Further, the higher percentage of binder blocks electrical flow between the active material particles, thus increasing resistance and reducing power density for the device. Therefore, in conventional coating processes it is a common practice that a “just enough” binder, for example about 5-10% of total solids, is added in the electrode formulation resulting in compromised electrode bonding force. Accordingly, powder particles easily flake off electrodes made by conventional coating methods.
Electrodes for power batteries require low ESR which requires a strong powder to metal (PM) bond and low electrical resistance between the current collector and the electrode film. FIG. 1 is a diagram illustrating a conventional electrode 100. Referring to FIG. 1, the conventional electrode 100 may include a current collector 110, a primer layer 120, and an electrode film 130. When the electrode active materials are not very conductive, electrode manufacturers coat the current collector 110 with a primer layer 120 having main components of conductive carbon and a binder that is specifically designed for bonding the conductive carbon particles to the metal current collector 110. After the primer layer 120 is dried, a layer of the electrode film 130 is coated on top of the primer layer 120. Thus, the electrodes 100 coated with the primer layer have at least three layers: 1) the inner-most layer of the current collector 110; 2) the primer layer 120 coated on top of the current collector 110; and 3) the electrode film layer 130 at the outside coated on top of the primer layer 120.
A significant amount of energy and time is needed to dry and remove the solvents, which adds the cost and cycle time to the manufacture of the product. For example, drying time for electrodes produced by conventional coating processes can take 12-24 hours at 120° C. In addition, for commercial applications an NMP recovery system must be in place during the drying process to recover evaporated NMP due to the high cost of the solvent and to reduce potential pollution caused by releasing NMP to the environment. The mandatory recovery system requires a large capital investment and is expensive to operate.
Less expensive and environmentally friendly solvents, such as aqueous based solvents, eliminate the need for the recovery system, but the electrodes still require the time and energy necessary for the long drying step. Furthermore, some active materials, such as lithium contained powders used in Li-ion battery cathodes, are highly sensitive to the aqueous based solvents. Extended soak times in the aqueous solvent required by conventional coating processes significantly damages the functionality of such active materials.
Manufacturing electrodes with dry particles coated on current collectors eliminates solvents and the disadvantages that come with using them. Conventionally, a variety of methods such as pulsed laser and sputtering deposition have been used for dry electrode manufacturing. These conventional methods of dry battery electrode manufacturing, however, suffer from very slow deposition rates and a need for high temperature annealing.
A dry painting method, also known as electrostatic spraying, has been proposed. The dry painting method consists of using a powder pick-up and dispensing unit and an electrostatic spray gun to charge the fluidized dry particles. After being charged, the dry particles will be drawn to the grounded current collector and deposited on the current collector. The dry painting method is a simple and low cost method; however, just as in the coating method, an excess amount of binder (or binders) is used in the electrode formulation for the same reasons. Further, since the powder particles have to be charged to a high voltage, a conductive active material, such as graphite which is widely used in the Li-ion battery industry, is difficult to paint onto the electrode by the dry painting method.
Accordingly, there is a need to make an electrode for energy storage devices without using NMP such that there is no need of a solvent recovery system and no need of a long drying process steps. Also, there is a need to make an electrode energy storage device with minimum amount of binder yet forming strong powder particle to particle bond and strong powder particle to current collector bond.